Blender 2.76b and Three.js exporter not showing in Addons list after install?
I checked my console output, apparently I had 2 versions of blender installed and I installed it to the wrong folder:

It outputs connect failed: No such file or directory
Read new prefs: /home/USER/.config/blender/2.76/config/userpref.blend

I didn't even know I had a .config folder, I put it in the ../config/scripts/addons folder and now it works great!